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Prop Question- 3 blade vs 2- Catto vs Sensenich

sgfreeman

Member
I am buidling a 7A and plan on using an IO-360 (Superior) engine. I am looking for some input from someone who has flown RV's using O-360's with a fixed pitch 3 blade and a fixed pitch 2 blade for a comparrison. I am leaning toward a Catto prop, but am undecided about 3 or 2 blade. I know 2 bladed props are suppose to be more efficeint and therefore cruise and climb are a little better, but how much? I also have read and been told that a 3-bladed prop is smoother and more quiet, especially with an O-360.

It would also be great if there was someone out there who could give a comparrison between a Catto prop and a Senisnich prop.

Thanks in advance for any feedback provided.

Scott Freeman
Marion, Virginia
 
After flying 60 or so hours behind my Catto 66 x 76, my current thoughts on the prop is that it is adequate. It could be better. How? Well it currently only spins up to 2620 max rpm! I am getting right at Van's numbers (197-198 mph) but not as fast as Craig said I'd be (205 mph cruise at 2700 RPM). I think it's pitched too much. With the extra RPM I bet it could be slightly faster. Craig did say I could send it back to be repitched for free but I haven't done that yet.

I do like my prop because it's extremely smooth and light.
 
I went from Sensenich wood (70 X 82) to the Catto 3-blade (66 X 74) a little over 2 years ago. I wanted somewhat of a climb prop since I operate out of 1500' strip in Texas. I have an O-320 with 9.5:1 compression putting out about 176 hp. My take off and climp improved by 17% and my cruise is the same as with the Sensenich. 201 mph TAS @ 75% power. MUCH quieter and smoother than even the wood Sensenich. I could never go back.
 
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Me either

I couldn't go back either. Yesterday at 7500' and 2600 RPM, we were trueing 197 or so, two average souls and full fuel. I figured around 65% and 9 GPH or a little over. Groundspeed at 221 mph not too shabby.

Both a 210 and Bonanza pilot refused to race for pinks :D

Ya gotta luv it!
 
Scott Will said:
Well it currently only spins up to 2620 max rpm!
I've got a 66X76 3-blade Catto on my O-360/ RV-6. If you are only getting 2620 RPM, then I suspect something is wrong. Tach not right, engine not making full power, something like that. I can wind mine up to about 2800.

Do you have all the fairings installed? What is your static RPM?
 
An old A & P and a teacher of aviation (WWII B-24 pilot and one of the original founders of the EAA) went over this question with me in detail, because I wanted to use a 3 bladed prop on my plane. Here is his explanation of the propeller situation.

First, he told me a single bladed prop is the best. It is always cutting through clean air, therefore it should be the most efficient. Balancing is the problem with this configuration. It is weird and not used much.

Two bladed props are the best compromise, if you can't use a single bladed prop. The efficiency is a little less, but the best overall situation.

Multiple blade propellers are used, mostly, where there is a need for a lot more horsepower and there is not enough ground clearance (e.g. diameter gets too large to use the available HP). The more blades the more efficiency is lost.

He strongly thought that two blades was best. It uses the available HP better. He was a big advocate for CS props, too. You just can't make a fixed pitch prop that works the best for take off and climb and then can, magically, switch to a prop that is the most efficient for cruise. A CS prop attempts to do both the best. The problem is usually cost. The CS units are a lot more money than a fixed pitch, but if you are planning on putting a lot of hours and miles on your plane, you can amortize the cost through fuel savings. This is what he told me, and I think it is worth repeating here. Any rebuttals or different opinions? This is always an interesting topic of discussion.
 
Ohhh I have all the fairings installed. My static runup is 2200+ right where it should be. I think it should top out a little higher in cruise. Engine is brand new and running great.
 
Not exactly

RV6ARoger said:
First, he told me a single bladed prop is the best. It is always cutting through clean air, therefore it should be the most efficient. Balancing is the problem with this configuration. It is weird and not used much.

Two bladed props are the best compromise, if you can't use a single bladed prop. The efficiency is a little less, but the best overall situation.
That is a very popularly held misconception. Actually in a static condition, that is correct. However, our airplanes are not in a static condition, hence the error. The propeller is carving a helical path thru the air. At 200mph and 2800 rpm on a 3 blade prop, each blade is 25" in front of the other at the same point of rotation. So each blade is traveling in "clean" air.

And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).
 
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My RV8, o-360, 3blade Catto, 2600rpm @ 7500 gives 174ktas (201mtas) @ 7500. This is about 68% @ 8gph.
 
RV6ARoger said:
An old A & P and a teacher of aviation (WWII B-24 pilot and one of the original founders of the EAA) went over this question with me in detail, because I wanted to use a 3 bladed prop on my plane. Here is his explanation of the propeller situation.

First, he told me a single bladed prop is the best. It is always cutting through clean air, therefore it should be the most efficient. Balancing is the problem with this configuration. It is weird and not used much.

Two bladed props are the best compromise, if you can't use a single bladed prop. The efficiency is a little less, but the best overall situation.

Multiple blade propellers are used, mostly, where there is a need for a lot more horsepower and there is not enough ground clearance (e.g. diameter gets too large to use the available HP). The more blades the more efficiency is lost.

He strongly thought that two blades was best. It uses the available HP better. He was a big advocate for CS props, too. You just can't make a fixed pitch prop that works the best for take off and climb and then can, magically, switch to a prop that is the most efficient for cruise. A CS prop attempts to do both the best. The problem is usually cost. The CS units are a lot more money than a fixed pitch, but if you are planning on putting a lot of hours and miles on your plane, you can amortize the cost through fuel savings. This is what he told me, and I think it is worth repeating here. Any rebuttals or different opinions? This is always an interesting topic of discussion.
Properly designed props of any number of blades are just as efficient. The absolute worst thing you could ever have is a single-blade prop. It only pulls on the side it is on, which means it would shake the **** out of the engine. That old wive's tale about the number of blades should be put to rest. My three-blade prop allowed "Phantom" to go 241 mph at Reno on 250 rpm less than the two-blade the previous year, which did 221 mph. This year "Phantom" did 252 mph with a four-blade on the same rpm as the two-blade that did 221 mph! That's like having 48% more power!
 
Catto 3 blade

Hi Scott
I have a Catto 66X72 3 balde on my RV4 @ 8000msl 2700 rpm 174 ktas
starting @ 4700 msl (Carson City airport) climb is 1600 fpm @ 80 kts 1350 gwt
static is 2300 rpm ish
 
Its all in the Math and Aerodynamics

RV8RIVETER said:
That is a very popularly held misconception. Actually in a static condition, that is correct. However, our airplanes are not in a static condition, hence the error. The propeller is carving a helical path thru the air. At 200mph and 2800 rpm on a 3 blade prop, each blade is 25" in front of the other at the same point of rotation. So each blade is traveling in "clean" air. I do agree 100% the airflow around a prop is very complex and changes for every flight condition (air speed, air density, power applied). The best way to make a determination is test. Test have shown two blades on RV's are faster than three. Regardless of your theory, the fact is the Sensenich and Hartzell (two blades, metal) are the fastest props you can buy for a RV.

And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).
It is not rumor or hearsay that props and number of blades affect efficiency and are related. Its all in the math, physics and aerodynamic of props. The basic rule is use the least number of blades to meet the design goals for engine HP, forward speed, altitudes, mission and airframe. If the math below does not make sense, than the empirical data is very clear. Adjacent blades DO affect each other and do not operate in clear air because its a "fur ball" of air flows, up and down wash and changes in relative air flow into and from the blade. The next blade is affected to some degree. Your 1/4" advance clean-air theory is too simplistic for 3D air flow around a prop.

Wade HP has EVERYTHING to do with props, number of blades, size (diameter) and efficiency. HP is related to BLADE loading or disk load and efficiency. It is like weight a wing has to lift, props have to absorb so much HP. Obviously if you have lots of HP, say 300 HP to 350 HP you start getting into a NEED for more than two blades.

Here are the key equations and coefficients:

Thrust
frm_ct.gif



Power
frm_cp.gif


Advance Ratio (J)
frm_vnd.gif


Efficiency
frm_eta.gif



where

v velocity m/s
D diameter m
n revolutions per second 1/s
ρ density of air kg/m?
P power W
T thrust N

You notice no blade area or airfoil data. This is simplified equations and Ct and Cp are usually calculated in testing and with more complicated formulas, but this shows the basic factors affecting efficiency of props, for more advance descriptions see the "Google" or:

http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion7/UnifiedPropulsion7.htm
http://www.allstar.fiu.edu/aero/BA-Background.htm


As Mel said it, there was some gains and losses but overall he likes the three blades. Also he did a wood to wood prop comparison. When you get into METAL props, which are thinner than wood, you are going to automatically be more efficient regardless of number of blades. Again there are pros and cons to each material and design.

It seems that people that are turned on with three blades don't want to accept that there ARE TRADE-OFFS. Three blades has advantages but TOP speed in NOT ONE OF THEM. You want lower noise, possibly better climb, better ground clearance and subjectively better looks and smoothness, than be happy with those positive attributes. However at high speed you give away some (I SAY SOME) efficiency. Is that efficiency a big deal? Well if you are Rocking 700HP or 1200HP turboprop no. If you want that extra 3-9 mph in cruise or top speed than its a big deal.

Also as mentioned three blades for our RV's tends to come with WOOD as a material, which is a negative on efficiency due to blade thickness being greater. Blades have a total velocity vector of almost super sonic or transonic. Airfoil thickness is critical. Metal props are thinner. You can get into advanced carbon-carbon blades, which are almost as thin as metal, but than they are so stiff, you lose that natural "smoothness" wood gives you.

Her is the bottom line, There is NO free lunch. Pick the attributes that turn you on. For some just looking cool on the ground is their thing. Others want every MPH they can get out of their RV and want an install and forget prop (ie metal). Some need to save weight on the nose (not RV7's which NEED weight, light props are negatives with parallel valve engines). The list of PROS and CONS is long, but it is a personal choice and there are no right / wrongs or absolutes, just trade offs. For the money, value and performance the Sensenich is the clear winner. For constant speed the Hartzell BA prop is a clear winner in my book; however I lean more towards absolute speed performance and not aesthetics, smoothness (which is subjective), maintenance, support (maintenance and engineering) and value. Support of the product is a BIG BIG factor for me. Regardless of NUMBER OF BLADES, boutique prop makers are not high volume and their business is balanced on one key person many times. This can cause less than stellar customer service due to the limitations of the one man show. Just be willing to deal with that. Sensenich or Hartzell can be worked on almost any where and you can get a new prop if needed, sometimes overnight. I doubt small prop companies can do that.
 
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All of these theoretical formulae of Ct, Cp, J, etc. do not address the really important aspects of propeller design and efficiency, namely: does the prop have a klunky shape where it enters the spinner or is it a correct airfoil at the correct helix angle. Is it sealed to the spinner or does it form a separate prop tip with loss of thrust. What is the tip Mach. Is the airfoil optimized to produce the best L/D. Is the blade Cl optimized to give the best blade loading. What is the thrust/torque ratio of each radius along the blade; is it constant or is most of the lift concentrated in the outer 20% of the blade so that the inner portion contributes drag only. One European outfit actually uses that as something to brag about, rather than hiding their heads in shame! Those are the formulae that are missing in the texts on propeller design!
 
Custom props and they know

elippse said:
All of these theoretical formulae of Ct, Cp, J, etc. do not address the really important aspects of propeller design and efficiency, namely: does the prop have a klunky shape where it enters the spinner or is it a correct airfoil at the correct helix angle. Is it sealed to the spinner or does it form a separate prop tip with loss of thrust. What is the tip Mach. Is the airfoil optimized to produce the best L/D. Is the blade Cl optimized to give the best blade loading. What is the thrust/torque ratio of each radius along the blade; is it constant or is most of the lift concentrated in the outer 20% of the blade so that the inner portion contributes drag only. One European outfit actually uses that as something to brag about, rather than hiding their heads in shame! Those are the formulae that are missing in the texts on propeller design!
I agree prop design verges on almost ART and a little black magic. That is why, "the proof is in the eating of the pudding". You don't know until you fly it. Airfoil area, diameter, twist distribution, chord, tip shape, plan form shape and so on are all interrelated variables that defy quantitative absolutes. It gets you 90% of the way, but 10% is that magic of designer. Like the RV has wings and all just like any plane, but the combo is just right and hits some magic balance of compromises.

Believe me Hartzell and Sensenich know as much or more about props than anyone. What every the european Prop company is doing is not new or unknown. Hartzell/Sensenich understand lift distribution varies along the prop span due to rotation. The fact is at the blade ROOT is un-aerodynamic in shape for one reason, strength and attachment to the HUB. Its just practical considerations.

Look at any GA prop near the hub, they have the aerodynamics of a 2x4 or baseball bat. It's just the way it is for any small plane props. Now look at a Hamilton Standard prop on later P-51's, which had an airfoil right up to to the spinner. The spinner was a huge diameter deal they could hide the blade attachment inside. Our spinner is not large enough in diameter and is just enough to hide the hub and blade shanks. You could make a bigger spinner, but it would change the whole cowl and plane.

Look at the evolution of props on the C130 military transport. First three blades, than 4 fat square paddle blades. Now I think its 5 or 6 scimitar blades (scimitar - curved like a turkish sword). The new C130 curved blades are no doubt an improvement in some areas of operation and not so much in others. In part they have them because manufacturing process allows them to do it. However I am sure they cost is huge. With fast turboprops they run into tip speed issues (which is build into analysis theories). In fact tip speed is one of those limiting factors that brackets the design. When you get into Mach 0.90 prop tips speed, you start to take a big efficiency hit. The scimitar blades have more sweep at the tip which improves high speed drag. Big turboprop wind makers are turning something like 1900 rpm, not 2,700 rpm, but their TAS is higher, HP is higher and they have larger diameters, which all increase tip speed. Therefore they have more blades because they need them, not because it is cool.

RV's are in the Mach 0.82-0.83 range at high speed full RPM. RVs don't need swept blades and if you could get them, they would cost more than the plane. At some point going flying is the best prop. For the money the Sensenich fixed prop is one of the best values. However I do admire the handmade wood or wood/fiberglass props, but they cost as much or more than a Sensenich. The advantage of a wood prop use to be they cost $600-$800. Those days are over. Whats a new Catto cost? $3,000?

There are several "theoretical" prop analysis approaches and design methods. That is why companies like Hartzell and Sensenich have an advantage, 50-80 years of experience each. They have developed their ability to correlate the theoretical analysis and design to actual performance of light plane props. The little guy whittling some wood in his shop can make a nice prop, but it is that last few percent, 2%, 3% or 4% that is key. They are not going to make 10 props and test all of them just for the RV.

Both Sensenich and Hartzell make special RV PROPS. These props are tailored to the airframes aerodynamics, a custom fit if you will. They did not get there just by analysis. Much of it was flight tested, so analytical and empirical data was correlated. Once that's done, the analysis and design are fine tuned. This is typical of engineering and aerospace. My criticism of other props was the blades sold to RV'ers where what they had in production for other (slower) planes.

That is the KEY people don't understand. Most other props are just approximate generic props. The old Hartzell HC2YK/F7666 was good for the Mooney and Piper Comanche, and it worked pretty good for the RV, which has similar performance and HP. However the blended airfoil is 3 mph faster. That does not sound like much, but that 1% or 1.5%, represents a breakthrough. Prop efficiency gains are measured in fractions of a percent. To get that you need to hone every prop aspect for that specific airframe. No other prop company makes a real RV prop in my opinion. Are the other one good or close enough? yea sure I guess, but I want that last 1.5%.
 
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GEE

George

Thanks for all the pretty formulas. :)

I never said 3 blades or 2 or 10 were best. I do disagree with the one blade statement, because we are not talking about a "static" environment. The "furball" you mention is a technically valid point, but it would be more at play on the non-leading blade(s) and how much would depend on forward speed and alot of blade design characteristics. I was only trying to keep things simple.

I guess should have made myself more clear on the power thing. I did not mean to imply hp was not relevent. Indeed it is, but what is that hp doing? Spinning the blade(s). So you want max hp to spin the blades at max rpm (I used 2700 as a practical Lyc number). The blades don't feel or know or care what hp it is. For sure the designer does. And for sure higher hp(and torque) the engine, the more Diameter blade you would be able to push, which would require adding blade numbers to keep the landing gear from being 15 ft long. But say you do not have more than 2 blades unless you are over 300hp is also too simplistic. Obviously there are a multitude of variables and factors at work; airfoil shape, thickness, diameter, air density, rpm ect,ect. I know it is a very complicated subject. I also know when learned people use the "black magic" term it means I(we) don't know all the pertinet variables to plug into a formula.

I also made no mention of trying to downplay your "beloved" Hartzell or Sensenich. They are very good at what they do. I do not buy into the notion that they "know all there is to know". We don't know all the other factors that go into prop manuf, like R/D budget constraints, the what we have is good vs would the improvement justify the cost, a 3rd or 4th blade may increase the cost of the prop enough to shrink the market share.

I will finish by agreeing that the "proof is in the pudding". Real world experimental data, beats formulas and theorys every time.
 
My 15% THICK, THREE blade design on the Jeff Lo-Chris Ferguson Reno biplane "Miss Gianna went 4.3% faster than their best-of-two THIN MacCauley metal props at the same rpm. That's like having 13.6% more power. And no, prop design is not a black art, it is purely and totally scientific. I put engine sea-level power, equivalent parasite drag area, wing span/area, and desired CL into my equations and out pops a design that I can predict to 1% or less what the performance will be in speed and rpm vs dalt. That's why I got into prop design from electronics; all of these cabinet-makers turned prop-makers would like you to believe that its a black art, known only to a select few sworn to secrecy. And if the big-three prop makers knew what they were doing, why is it they keep turning out the same old shapes dressed-up with "swept-back tips", "blended airfoils" and other hype. The prop can and will generate thrust all the way to the spinner when properly designed!
 
I feel the need for speed and want 4.3% faster

elippse said:
My 15% THICK, THREE blade design on the Jeff Lo-Chris Ferguson Reno biplane "Miss Gianna went 4.3% faster than their best-of-two THIN MacCauley metal props at the same rpm. That's like having 13.6% more power. And no, prop design is not a black art, it is purely and totally scientific. I put engine sea-level power, equivalent parasite drag area, wing span/area, and desired CL into my equations and out pops a design that I can predict to 1% or less what the performance will be in speed and rpm vs dalt. That's why I got into prop design from electronics; all of these cabinet-makers turned prop-makers would like you to believe that its a black art, known only to a select few sworn to secrecy. And if the big-three prop makers knew what they were doing, why is it they keep turning out the same old shapes dressed-up with "swept-back tips", "blended airfoils" and other hype. The prop can and will generate thrust all the way to the spinner when properly designed!
So when I can get a prop from you for my O-320 160 Lycoming for my RV-4?
 
Hopefully, very soon. Vari-prop will soon make a three-blade CS of my design, and I have a prop I loaned to a fellow in Wichita to try on his RV-6 so I can get parasite drag data. He should get the prop tomorrow, and, if the weather stays good, maybe he'll have some comparitive test data this weekend. This was my first prop, and was not yet an optimized design, but I have it modeled im my equations, and so can use it for data gathering. He has contacted a fixed-pitch prop maker about making him a three-blade if all goes well!
 
One other thing I failed to mention. Along with Jeff Lo's composite-over-wood prop being thicker than his metal prop and having three blades rather than two, it was also several inches less in diameter! Obviously we have a breach in the space-time continuum that is reversing all of our mathematics and well known facts! Where will this end? Someone should contact George Noory or Art Bell at "Coast-to-Coast AM" and tell them to get the word out so that this trend may be reversed. Global warming will have to take a back seat to this new threat!
 
well that settles it!

I've just started the wings on my RV9A but I've been thinking about and trying to learn as much as possible about engines, props, intruments for the day when I have to make these decisions.

This is very complicated stuff.

I think I'll just sacrifice a goat, chant some funny latin words and burn some incense in hopes the airplane gods will grace me with good judgement!

Dave (the confused) :D
 
Catto 3-Blade (66x76) 4 way gps, 2660 rpm, a few weeks ago gave me 198mph @ 8500msl.

Winds aloft will be calm here Saturday, I'll record some new numbers.
 
elippse said:
One other thing I failed to mention. Along with Jeff Lo's composite-over-wood prop being thicker than his metal prop and having three blades rather than two, it was also several inches less in diameter! Obviously we have a breach in the space-time continuum that is reversing all of our mathematics and well known facts! Where will this end? Someone should contact George Noory or Art Bell at "Coast-to-Coast AM" and tell them to get the word out so that this trend may be reversed. Global warming will have to take a back seat to this new threat!

Paul,

What ever happened with this data? Not that I need an RV prop, but I would like to see your props make the mainstream. They are indeed awesome as exhibited by the performance that i've seen on your personal airplane and the planes and Reno that fly your airfoils.

Let us know,
 
Is this reasonable?

elippse said:
My 15% THICK, THREE blade design on the Jeff Lo-Chris Ferguson Reno biplane "Miss Gianna went 4.3% faster than their best-of-two THIN MacCauley metal props at the same rpm. That's like having 13.6% more power. ....
Assuming for the sake of discussion that the McCauley was 85% efficient, that would make your prop 96.56% efficient. If that's not reasonable, then what do you suppose the two efficiencies are? If the McCauley was not at 85%, is that no longer a valid rule of thumb? I am having trouble understanding this data in context of what the community uses as operating assumptions.
 
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I would guess that because of the high tip speed these props run at the high rpm they turn that their efficiencies are probably more in the 75% to 80% range. That would put mine more like 85% to 91%. Keep in mind that when a long prop is operating with the outer portion of the blade at high subsonic Mach, the much higher drag is multiplied through the long radius to reflect a high torque load to the engine. At M0.85 the profile drag Cd is about eight times greater than at M0.65. Since the prop has little-to-no lift near the tip, it can only have drag, and that will be proportional to the blade area. So wide tip-chord at high Mach equals big loss! My data seems to indicate that the three-blade on my Lancair has a cruise efficiency of about 90%-92%. Some have said it is theoretically impossible to exceed 90% efficiency, but some recent thought holds that it could go as high as 95% maximum efficiency. I'll leave that to the aeronautical theoreticians to battle over. Those hypotheses will have to wait until someone actually runs some rigorous testing on some kind of advanced blade profile. The nice thing about the Reno results is that there isn't too much you can argue with over the race results!
 
That is what its all about

Daver said:
I've just started the wings on my RV9A but I've been thinking about and trying to learn as much as possible about engines, props, instruments for the day when I have to make these decisions.

This is very complicated stuff.

I think I'll just sacrifice a goat, chant some funny latin words and burn some incense in hopes the airplane gods will grace me with good judgement! Dave (the confused) :D
Enjoy the Ride Dave, we all have been there and that is what home building is all about. Not only do you end up with a beautiful plane but you get learn a lot. It is not complicated, just take small bites. The fog will lift. :D Some old golden rules:

Keep it simple
Build it per plans
Keep it light weight
Airplanes are compramises, if its better in way way, its a loss in others.
 
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Paul - follow up question

OK, the high tip speed explanation makes good sense without any rigorous testing. Does your design have big advantages at, for instance, 2700 rpm with a 72-74" prop? Thanks.
 
BASIC C/D LESSON!!!!!

And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).[/QUOTE]

ACTUALLY IT HAS EVERYTHING TO DO WITH IT!:rolleyes:

The true reason for efficiency loss as you add blades is this and this alone. You are adding more frontal area (leading edge of blade) and also increasing the total wetted surface area. These two items account for the only differences in efficiency. This extra drag consumes a considerable amount of power that could be used to create thrust and power the plane. For instance, were you to keep adding blades you would soon need to start taking out pitch in order to maintain the RPM. Eventually you would have a propeller with no pitch and many blades that would consume all available power just to spin it through the air and create zero thrust. Keep in mind the leading edge tip speeds are approaching 650 mph on each blade and that requires considerable power to push through the air. Regards All, Allan
 
And if you think about it, what relationship exactly does hp have with blades? As long as the engine can turn the blades to the rpm you want, say 2700-2800, then what more can it do? The blade only knows it is going 2800 rpm, it doesn't know what hp is pushing it. So hp is more related to blade pitch, length, width and number (only so much as you want it to spin at a given rpm).

ACTUALLY IT HAS EVERYTHING TO DO WITH IT!:rolleyes:

The true reason for efficiency loss as you add blades is this and this alone. You are adding more frontal area (leading edge of blade) and also increasing the total wetted surface area. These two items account for the only differences in efficiency. This extra drag consumes a considerable amount of power that could be used to create thrust and power the plane. For instance, were you to keep adding blades you would soon need to start taking out pitch in order to maintain the RPM. Eventually you would have a propeller with no pitch and many blades that would consume all available power just to spin it through the air and create zero thrust. Keep in mind the leading edge tip speeds are approaching 650 mph on each blade and that requires considerable power to push through the air. Regards All, Allan[/QUOTE]

Sorry, but I have to disagree with you on all points. When you add more blades for a given speed and rpm, you do not decrease the pitch, you decrease to chord. There is only one loss on a propeller blade and that is to due to parasite drag.
Wide tips at high Mach have a higher parasite drag coefficient so not only do they have higher drag, due the the greater CD and area, that drag force is multiplied by the radius to give a higher loss-horsepower. Keep in mind, torque times rpm / 5252 = horsepower. A 3.5" wide tip on a two-blade prop turning 2700 rpm at sea level at 135 mph will have a 3.5 HP loss for just the last inch of span due to tip drag. If your prop has wide tips you're throwing away a lot of HP, especially if they have a round planform.
A loss drag half-way out on the blade will only consume 1/2 the horsepower as will tha same loss drag at the tip. Multiple blades that have streamlined shapes following the correct pitch angle all the way into and sealed to the spinner are every bit as efficient in cruise as a two-blade. If what you were saying was true, which it isn't, then Tom Aberle's Phantom biplane would have gone slower in going from a two-blade (221 mph) to a three-blade (241 mph) and even slower still when going from the three-blade to a four-blade (260.805 mph). Actually multiple blade props can be more efficient since they can be made smaller in diameter and have the same efficient mass flow which reduces tip Mach and loss drag.
Unfortunately, your thinking process has been poisoned by all of the nonsense that has been printed about propeller blades by people who didn't have the first idea of what they were talking about. They spoke of blade-to-blade interference when in reality each blade follows its own individual helical path through the air.
Just look at the pix of the C-130J that has eight-blade props with narrowed tips or the A-400M with similar blades. The old ideas are slowly melting into the past but it will take a long time for them to be purged entirely since they have been around and have appeared in books for so long. Where's Fahrenheit 451 when you need it?
 
I was assuming all dimensions remaining the same for comparison purposes only. This was an attempt to simplify the answer to the original question, not to try reinventing the propeller! I am a design engineer for Garrett with 46 years experience, am very capable of running the numbers as well as accessing the worlds largest database on the subject. The question was simple and only requires a simple answer. Over complicating or introducing complex mathematical equations that are Greek to most people on this forum serve no purpose and only adds to the confusion. Regards, Allan
 
Does anyone know if Paul ever made props? He last posted on here in 2011. There was some great ideas bouncing around in this thread and then it just stopped...
 
After reading Paul's stuff, I trimmed down the tips on a 3-blade Performance Prop that did not give me enough RPM. Trimmed it from the tip to about 8" in, that is, I reduced the chord and thinned the tips. I got 125 more RPM and an increase in speed.

Earlier, I damaged a tip on this same prop and sawed off about 1.25" of the original blades. That had very little effect.

http://forum.canardaviation.com/showthread.php?t=5170
 
Craig too

Craig Catto is trimming the tips too, down to almost a point..there's a brand new one on a -4 in my hangar...a work of art.

Best,
 
Does anyone know if Paul ever made props? He last posted on here in 2011. There was some great ideas bouncing around in this thread and then it just stopped...
Sadly, Mr. Lipps passed away a few years ago. His Ellipse props were very unique in design. There is a gentleman in Kansas who retains the rights to manufacture the Ellipse props designed by Paul Lipps. I cannot recall his name but he does fly an RV6 with an Ellipse prop. He has also designed his wingtips in the similar shape as the prop tips. It is a unique looking RV6 when you see it.

Here is a quick google search result of some discussions concerning Paul Lipps and his props:
https://www.google.com/search?q=ellipse+props&ie=utf-8&oe=utf-8#q=paul+lipps+ellipse+props
and some pics showing some of his props:
https://www.google.com/search?q=paul+lipps+ellipse+props&sa=N&biw=1344&bih=684&tbm=isch&tbo=u&source=univ&ved=0ahUKEwje8K396qnKAhUP8GMKHWbLBY44ChDsCQg_
 
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Not to over-simplify the answer but I am in the same situation and going with a 3 blade Catto simply for the ground clearance. Pulling the engine for an inspection will slow me down far more than the 3rd blade (if it slows me at all).
 
Pierre, can we get a picture?
I am not Pierre but here is a side view shot of the newest 3-blade Catto design. I started with an earlier prop design from Catto, which you can see in the second picture. Perhaps this side view will help with showing a profile of the blade shape:
side%252520view%252520of%252520prop.jpg


Here is a frontal view of my plane with the original 3-blade Catto. Although this picture is an older one in which I had Catto's earlier design, I thought it valuable to post in the event you were wishing to see the overall appearance of the 3-blade prop:
IMG_7864.JPG


The appearance of the two generations of props is not initially noticeable until you look up close and study the details. The overall thickness of the blades is much thinner. The tips curve much more than the earlier generation blades. The tips come to more of a point as the curve reaches the end. Subtle differences that really do make a difference in performance.
 
Hello:

I just did a WOT test on my RV-8A to help the Catto peeps with my (potential) prop order. I should note that my nose gear fairing is currently removed. Anyway, the stats:

166Kts TAS @8,500' 2650 RPM, 2 bladed Sensenich FP metal prop.

This seems a tad slow.... What should I expect from a 3-bladed Catto? I'm leaning towards a prop slightly pitched for speed over climb - my home field has 2 long paved runways, and most airports I visit are at least 2000.' However, I'm thinking about joining in on formation flying, and those guys usually have CS (I'm not going to make that jump just yet.)

Thx.

UPDATED INFO: Also, developing 73% power, full rich, 29F OAT.
 
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Not an -8 but my O-360 powered -9 with a 2 bladed Catto will true out at 174 kts at 8,000' DA. All in and leaned for best power.

While I have a longer wing and no nose wheel, the speeds should be closer than that.

The 2,000' runway should not be an issue.

When doing formation work, just make sure lead knows and if they are any good, they will not close the throttle on you so they can watch you go sailing on by.

No worries, enjoy the Catto.
 
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Take-off distance for an RV-8A with a 180 HP and a constant speed prop is spec'ed at 575' so figure on a fixed Catto doing about 650-700 at gross weight (still very good all things considered)
 
Don't underestimate the value of the nose wheel fairing. It's got to be worth several knots at least.
I asked Catto for Van's numbers on my RV8 with a Superior IO360. I get 178 ktas at 2700 rpm's from my 3 blade 68x75 at 8000 feet da.
 
Don't underestimate the value of the nose wheel fairing. It's got to be worth several knots at least.
I asked Catto for Van's numbers on my RV8 with a Superior IO360. I get 178 ktas at 2700 rpm's from my 3 blade 68x75 at 8000 feet da.

The fairing is off because I installed the redesigned fork / shortened nose gear leg late last summer, and haven't modified the fairing to accept the new bracket arrangement. It's on the list though...
 
Paul's old entries?

It was some time ago but the last time I checked into this forum I found Paul's old entries . . . but now either I'm typing a name wrong or Paul's entries have been moved into some less accessible area?

Anybody can direct me please?

Mike
 
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